The present invention is drawn to an angular rate sensor of the type utilizing an oscillating resonating element. More specifically, the present invention is drawn to the shape and placement of actuators and pick-offs upon resonating elements of rate gyroscopes.
Rate gyroscopes operate on the principle of inertia. Standing waves are excited in a resonating element to produce a desired mode of oscillation having a predetermined number of nodes. The oscillations have an amplitude, a frequency, and an inherent oscillatory inertia that is independent of the linear and rotational inertia of the gyroscope itself. When the resonating element is rotated about its sensing axis, the oscillations will in large part maintain their absolute spatial orientation. However, in maintaining their absolute spatial orientation, the nodes that define the desired mode of oscillation will rotate with respect to the physical structure of the resonating element. This rotation of the nodes is proportional to the physical rotation applied to the resonating element. Taking advantage of this phenomena, it is possible to measure the rate of rotation and determine the magnitude and direction of the rotation that the resonating element has been subjected to.
Solid state gyroscopes based on the principle described above are capable of sensing rotation only, and then only about a single axis. To obtain information sufficient to determine the relative attitude of a body, it is necessary to group at least three such gyroscopes in an orthogonal relationship covering the x, y, and z Cartesian axes.
The terms xe2x80x9cgyroscopexe2x80x9d and xe2x80x9cangular rate sensorxe2x80x9d as used herein are interchangeable and refer to both spinning and oscillating or vibrating type devices. One well known type of angular rate sensor comprises the use of piezoelectric ceramic crystals in a paired tuning fork arrangement. Examples of this type of angular rate sensor are shown in U.S. Pat. Nos. 4,628,7734 to Watson and 4,671,112 to Kimura. In this type of sensor a pair of drive elements are energized to induce a controlled vibration therein. The drive elements are arranged such that the oscillations induced are in a single plane. Sensing elements are coupled to the ends of the drive elements and oscillate along with the drive elements in the single plane. However, the sensing elements are arranged so that flexure of the sensing elements will take place only in a plane perpendicular to the plane of vibration of the driving elements. The application of a rotational force to the vibrating sensor elements in the perpendicular plane induces a sensed output signal that may be monitored and filtered to characterize the angular rate of change of the sensing object to which the sensing elements are mounted. Though the tuning fork type of angular rate sensor attempts to isolate the sensing elements from the drive elements by rotating the sensing elements 90xe2x96xa1 from the drive elements, small bending forces due to the oscillation of the drive elements are imposed upon the sensing elements. These undesirable bending forces create voltage signals which may degrade the signal to noise ratio of the voltage output of the sensing elements and may indicate falsely that the angular rate sensor is being rotated about its sensitive axis.
Another type of angular rate sensor utilizes a cup or bell shaped resonator which is forced to oscillate in known manner. One such sensor is shown in U.S. Pat. No. 5,218,867 to Varnham, et al. See FIGS. 1-3. The cup portion of the Varnham resonator is supported upon a stem which is in turn secured to the chassis of the sensor. Varnham utilizes a pair of actuators arranged at an angle of 45xe2x96xa1 to one another to induce a desired mode of oscillation in the resonator. The resonator itself is fabricated from a piezoelectric ceramic material and the actuators are thin or thick film conductive materials that are applied directly to the wall of the resonator in a known manner. In order to sense a rate of rotation, the Varnham device provides a pair of pick-offs, identical in construction to the actuators and applied to the resonator in diametric opposition to the pair of actuators. An actuator drive network acts through the actuators to impose a phase locked voltage waveform upon the resonator, thereby causing the resonator to assume a desired mode of oscillation. The pick-offs sense variations in the desired mode of oscillation caused by angular rotation of the sensor. The signals from the pick-offs are demodulated using the imposed driving voltage waveform. The resulting signal is proportional to the angular rate of rotation of the sensor and by integrating the resulting signal over time, one can determine the actual angle through which the sensor has rotated. The angle of rotation is, in turn, used by the actuator drive network to modify the waveform being imposed upon the resonator to bring the resonator back to the desired mode of oscillation.
Problems with angular rate sensors of the type patented by Varnham include a relatively low Q value, low sensitivity, and low accuracy. For instance, the actuators and pick-offs of prior art devices such as the Varnham device, are uniformly large patches of conductive material applied to the resonator in a manner such that the actuators and pick-offs span a wide range of stress gradients in the resonator walls. Because piezoelectric voltages are generally proportional to the stress in a piezoelectric material, a voltage applied across a number of stress gradients causes the areas of differing stress within the piezoelectric material to work against one another, thereby reducing the Q value of the resonator. Likewise, a voltage measured across a wide-ranging stress gradient is more likely to be an average of the voltages produced in the resonator at each of the stress gradients that a pick-off crosses.
In addition, the application of actuators and pick-offs across stress gradients, in combination with non-uniform voltage responses in the piezoelectric materials, may make it more difficult to force the resonator to oscillate in its desired mode. In order to ensure the proper oscillation, much more energy is expended in the correction of the vibrations, thereby lowering the Q value of the resonator. The Q value of a vibrating system is the ratio of the magnitude of the total energy of a vibrating system to the magnitude of the energy added to the system during each oscillatory cycle.
The large size of the conductive patches of the pick-offs contributes to the low accuracy of rate gyroscopes of the type patented by Varnham. FIG. 3 illustrates prior art pick-off and actuator conductors C having large surface areas. Piezoelectric materials are not uniform in their voltage response and therefore it is frequently the case that a pick-off having a large surface area will sense net voltages skewed by an uneven voltage response of the piezoelectric material. The larger the area of coverage of the pick-off, the more likely it is that the voltages sensed by the pick-offs will comprise a signal due to uneven voltage response of the piezoelectric material of the resonator. And because the actual voltages sensed by the pick-offs are quite small, voltage signal components due to uneven voltage responses frequently alter the signal to noise ratio of the sensed voltages to an extent that makes it difficult to determine accurately the rate and magnitude of rotation of the gyroscope. Further more, because it is also frequently the case that the voltage response of respective areas of the piezoelectric materials that make up a resonator may vary independently with changes in the ambient temperature of the operating environment of the gyroscope, the noise to signal ratio of the sensed voltages may become further degraded.
In general, piezoelectric materials are made up of many individual crystals that have been sintered together and given a particular polarity by the application of a strong DC voltage. Where this polarization is performed over a discrete area of the piezoelectric material, such as over the surface of the resonator covered by the conductive patches of the actuators and pick-offs, the polarization of the material at the edges of the discrete area will not be in the desired direction and will therefore generate irregular voltage responses. In addition, it is not uncommon that the piezoelectric material of the resonator will be subject to irregular stresses or flexure. The combination of irregular stresses or flexure with uneven edge polarization, may cause severe fluctuations in the accuracy and sensitivity of the angular rate sensor and may also lower the Q value of the system.
In addition to the problems mentioned above, it is known to make electrical connections between actuators and pick-offs on a resonator and the associated sensing and filtering electronics, using fine wires as connectors. See FIG. 1. These wires are connected at each end but otherwise unsupported therebetween. The wire used in making these connections must be extremely fine so as to avoid interfering with the vibrations set up in the resonator. The small size of the connection wires makes them weak and prone to frequent failure due to applied forces and internal stresses resulting from the ultrasonic wedge bonding processes that are typically used to make such small electrical connections. In addition, the use of solder or solder-like materials on the resonator at the physical contact between the wire and the resonator tend to interfere with the inducement and maintenance of the desired mode of oscillation, thereby lowering the Q factor of the system.
Therefore, it is an objective of the present invention to provide a uniformly polarized piezoelectric resonator. It is another object of the present invention to improve the accuracy and Q value of the resonator by providing a plurality of actuators that are contoured to conform to areas of substantially uniform stress in the walls of the resonator and which are located on the resonator so as to maximize the flexure of the resonator wall per unit volt applied to the resonator. Similarly, it is yet another object of the invention to suppress unwanted modes of oscillation through the proper arrangement of actuators and pick-offs on the piezoelectric resonator. Another objective of the present invention is to provide a pick-off structure which minimizes error due to undesirable stresses and deformations present in a resonating element. Another objective is to reduce the inherent variations in output voltage sensed at the nodal pick-offs due to fluctuations in the environmental conditions in which the gyroscope is operating. Yet another objective is to provide more reliable electrical connections between the actuators and pick-offs and the electronics used to filter and process the electrical signals received from and sent to the actuators and pick-offs, respectively.
With the aforesaid background in mind, improved pick-off and actuator conductors have been developed which minimize error in the angular rate of change reported by an angular rate sensing gyroscope. Furthermore, angular rate sensing gyroscopes incorporating the present invention have a more uniform voltage response and are provided with conductive leads that are relatively resistant to damage.
An angular rate sensing gyroscope constructed according to the present invention comprises a resonating element that is arranged and constructed to output voltage signals proportional to a level of stress induced therein, means for imposing a predetermined mode of oscillation upon the resonating element, a voltage pick-off conductor on the surface of the resonating element that is arranged and constructed to sense stress-induced voltage signals outputted by the resonating element, and means for processing the voltage signals sensed by the pick-off conductor. The pick-off is applied to an area of the surface of the resonating element where the stress in the shell wall is minimal and preferably substantially zero when the gyroscope is rotationally stationary. Consequently, any voltage signals sensed by the pick-off conductor are indicative of the rate at which the gyroscope is rotating.
A resonating element according to the present invention is characterized by the ability to vibrate in a predetermined mode of oscillation defined by a plurality of stable nodes and anti-nodes. Actuator conductors of the present invention are applied to the surface of said resonating element substantially at the anti-nodes and pick-off conductors are applied substantially at the nodes. The advantageous arrangement of the actuator and pick-off conductors on the anti-nodes and nodes, respectively, results in a more sensitive and efficient angular rate sensing gyroscope.
The actuator conductors of the present invention are applied to the resonating element at predetermined locations upon the surface of the resonating element that are defined by boundaries that are congruent with areas of the resonating element that are subject to substantially uniform levels of stress when the gyroscope is rotationally stationary. Alternatively, the areas to which the actuator conductors are applied are demarcated by at least one stress gradient line that defines an area of substantially uniform stress present in the resonating element when the gyroscope is rotationally stationary. Essentially, the edges of the actuator conductors are congruent with the stress gradient lines that identify areas of substantially uniform stress in the resonating element. Often it is helpful for at least one of the actuator conductors to comprise two vertically symmetrical halves. These symmetrical halves are electrically isolated from one another and are independently electrically connected to a drive circuit that is constructed and arranged to apply a predetermined sequence of voltage signals to the resonating element through the actuators so as to impose a predetermined mode of oscillation upon said resonating element.
Placement of the pick-off conductors at the nodes of the resonating element ensures that the pick-off conductors will sense a net voltage signal of substantially zero when the gyroscope is rotationally stationary. But where due to geometric or voltage response discontinuities the net voltage signal sensed by the pick-off conductor when the gyroscope is rotationally stationary is not substantially zero, a balancing conductor may be applied to the surface of the resonating element in conductive communication with the pick-off conductor. Balancing conductors are arranged and constructed to zero any net voltage signals sensed by the voltage pick-off conductor when the resonating element of the gyroscope is rotationally stationary.
The resonating element may be any of a number of suitable shapes. Specific examples of resonating elements include, but are not limited to, cylinder-, ring-, and bar-shaped structures. The bar-shaped structures that may be used as a resonating element have a polygonal cross section. One particular example of a suitable bar-shaped resonating element is a triangular prism having three longitudinal sides with each longitudinal side having applied thereto a conductive element. In this example, two of the three conductive elements are used as pick-off conductors and the third is the actuator conductor. Another example of a suitable resonating element is a curvilinear axi-symmetric shell fashioned from a piezoelectric material.
The present invention may also be adapted for use with an angular rate sensing gyroscope of a type comprising a ring suspended from a support structure in a magnetic field by a plurality of leg members. This ring shaped resonating element is capable of vibrating at a resonant frequency that is defined by a plurality of vibratory nodes and anti-nodes as is more completely described below. The ring is further provided with a plurality of pick-off conductors that are arranged to sense electrical currents indicative of the rate of rotation of the gyroscope. These rotation indicating currents are induced in the pick-off conductors by movement of the ring and conductors through the magnetic field when the ring is deflected by rotation of the gyroscope. A plurality of actuator conductors are also arranged on the ring so as to pass currents through the magnetic field, thereby inducing resonant vibrations in the ring. In such a rate sensing gyroscope, the present invention embodies an improvement which comprises supporting the ring from a plurality of pairs of leg members. The leg members. are located adjacent to and symmetrically bracket the nodes of the ring. Pick-off conductors are arranged upon the leg members so as to form a loop, each pick-off conductor being applied down one of the leg members of a pair of leg members, across the portion of the ring intermediate the pair of leg members, and up the remaining leg member of the pair of leg members. This arrangement advantageously centers the portions of the pick-off conductors on the ring symmetrically about the respective nodes of the ring. Likewise, a plurality of actuator conductors are arranged in a loop, being disposed down a leg member of a first pair of leg members, along the ring intermediate the first pair of leg members and a second pair of leg members, and up a leg member of the second pair of leg members nearest the first pair of leg members. This arrangement also permits the actuator conductors to be centered symmetrically about the respective anti-nodes of the ring. The respective conductor loops formed by the pick-off and actuator conductors are, in turn, electrically connected to circuit means for operating the gyroscope.
In this embodiment of the present invention, each node and anti-node of the ring may be provided with a pick-off conductor or actuator conductor, respectively. However, there is no requirement that each of the nodes and anti-nodes have a conductor associated therewith. Furthermore, it may be desirable to extend the pick-off conductors and actuator conductors around substantially the entire circumference of said ring, though again there is no such absolute requirement
In a tuning fork type angular rate sensing gyroscope composed of vibrator components which include a pair of parallel piezoelectric drive elements and a pair of parallel piezoelectric sensing elements joined together in respectively orthogonal planes, a plurality of leads electrically connected to the drive and detection elements, and a plurality of lead terminals electrically connected to the leads, a voltage pick-off conductor according to the present invention is disposed on the surfaces of each of the sensing elements. These pick-off conductors are arranged and constructed to sense stress-induced voltage signals outputted by the resonating element that is indicative of a rate of rotation of the angular rate sensing gyroscope. The pick-off conductors are applied to areas of the surface of the sensing elements that are subject to substantially zero stress when the angular rate sensor is rotationally stationary The voltage pick-off conductors provide electrical pathways from the sensing elements to the leads.
In another embodiment of the present invention, a resonating element having a polygonal cross-section has a predetermined number of improved conductive elements applied to the sides or faces thereof. The conductive elements are applied to the resonating element at areas of the sides or faces that are subject to drive motion stress which, when differentially sensed, is zero when angular rate sensing gyroscope is rotationally stationary. The conductive elements of this embodiment may further be provided with a voltage balancing conductor applied to the resonating element in electrical communication with the conductive elements so as to zero net voltage signals sensed by said conductive elements when the angular rate sensing gyroscope is rotationally stationary. It is important to note that differential sensing using conductive elements that are applied to substantially the entire length of the resonating element tends to damp out uniformly applied disturbances to the resonating element such as vibrations and magnetic fields.
Another manner of ensuring that a resonating element will oscillate in a desired mode is to physically damp out unwanted modes of oscillation. This may be accomplished by altering the geometry of the resonating element at predetermined locations upon the element. This manner of physical damping of oscillations is particularly, but not exclusively, applicable to axi-symmetric type resonating elements such as the ring and the cylinder-shaped elements. With regard to the ring- and cylinder-shaped resonating elements, the specific means of physically damping out unwanted modes of oscillation may comprise thickening the walls or cross sections of these resonating elements at the anti-nodes thereof.
Rate sensing gyroscopes are more accurate when the piezoelectric material of the resonating element has a uniform voltage response. A method of improving the uniform voltage response of a piezoelectric resonating element at a predetermined location of a solid resonating element having first and second opposing surfaces begins with the step of applying a thick or thin film conductor to the entire first surface of the resonating element. Next, an applied film conductor is applied to the entire second surface of the resonating element. The respective applied film conductors are then connected to a DC voltage source which applies a DC voltage of predetermined strength across the respective applied film conductors so as to uniformly modify the voltage response of the piezoelectric material of the resonating element over substantially the entire area of the piezoelectric material located between the respective applied film conductors. Finally, predetermined portions of the respective applied film conductors are removed to create a plurality of discrete applied film conductors arranged upon one or both of the surfaces of the resonating element as described above.
The useful life of a rate gyroscope comprising an axisymmetrical resonating element is greatly improved by providing a plurality of applied film conductor leads which extend from each of the actuator conductors and pick-off conductors arranged upon the surface of the resonating element, to the base portion of the resonating element. The applied film conductor leads electrically connect the actuator conductors and pick-off conductors to circuitry for operating the angular rate sensing gyroscope. The use of applied film conductor leads in the place of fine wires reduces the amount of failures due to stress fracture of the wires.
These and other objects and advantages of the invention will become readily apparent as the following description is read in conjunction with the accompanying drawings wherein like reference numerals have been utilized to designate like elements throughout t he several views wherever possible.